CMOS devices contain both n- and p- channel field effect transistors (FET) and form the basis of integrated circuits. These transistors are metal oxide-based semiconductor devices which include a source and drain region and an insulated gate in between. As the density of integrated circuits and performance increases, the dimensions of the transistors have to be reduced. As a result the thickness of the insulated gate dielectric layer has to be made smaller. With regard to gate dielectrics, one of the desirable features of the dielectric layer is that it couple the overlying gate electrode to the underlying channel so that the channel is responsive to the stimulus applied to the gate. In this regard it is desirable for that dielectric to have a high dielectric constant commonly known as high K. Silicon dioxide has been by far, the most common and effective gate insulator used in making integrated circuits. This has a very high level of integrity and, in particular, is able to be made with a very low defect density. The result is that the silicon dioxide operates very effectively causing devices to have low current leakage. Unfortunately, the leakage current increases dramatically with reduced thickness of the gate dielectric. For example, SiO2 having a thickness of less than 20Å results in unacceptable leakage current and degraded device performance. Accordingly there exists a need to replace the SiO2 in CMOS devices. Leakage currents can be reduced by having a thicker high K layer having reduced equivalent (SiO2) oxide thickness.
One of the characteristics that is desirable for the high K dielectric is that it be amorphous. It must remain amorphous for its entire life including during manufacturing and subsequently during functional operation as part of the completed integrated circuit. Many of the alternative high K dielectrics have sufficiently high K and sufficient integrity at time of deposition, but over subsequent processing steps and the heating that is associated with that, the result is crystallizing of these films. These films that are so crystallized are not perfectly crystallized throughout their entire length and width but have areas known as grain boundaries between the crystalline structures that are formed. These grain boundaries are areas of leakage and other problems that affect electrical performance.
Currently there is much work being done in developing high K dielectrics that have a higher dielectric constant than that of silicon oxide. There are a number of those, but one of the advantages of silicon oxide is its high band gap and low interface state density with silicon which results in it being a very effective insulator. Thus, many of the materials being developed for high K purposes have been found to have problems because they do not have a high enough band gap or because they are difficult to make with enough integrity to prevent current leakage through the dielectric. Additional problems remain unresolved such as thermal stability with the silicon substrate and gate electrode, fermi level pinning at the oxide/metal interface and scaling. Even though amorphous materials including Hr-based and Zr-based oxides are being investigated, there appears to be no clear solution since there are outstanding problems with these materials when integrating into a CMOS flow. Also, these materials recrystallized during the high temperature steps during the manufacturing process. La-based oxide materials can potentially be used as a high K dielectric for Si CMOS devices. Such oxides have higher dielectric constant than SiO2 and are predicted to be thermodynamically stable in contact with silicon.
An alternative to amorphous is monocrystalline films. In theory, high K dielectric films can be made typically monocrystalline, although difficulties exist. One such difficulty is matching the crystalline structure of the film with that of the underlying semiconductor, typically silicon, as well as during the formation process that it be in fact perfectly formed. Epitaxial layers, that is layers that are monocrystalline, are known in the industry. Silicon can be made epitaxially. One of the techniques by which very thin films can be put down in a monocrystalline form is molecular beam epitaxy. Even with using MBE technology there is still the difficulty of ensuring defect free films.
In developing new high K dielectrics there is also another potential problem of having too high of a dielectric constant. If the dielectric constant is too high, there is an effect that is called fringing field effect which adversely affects the performance of the transistor. This has to do with excessive coupling between the gate and the source/drain. Thus, the materials that are being developed desirably have a range typically between 20 and 40 for the dielectric constant. This range may change somewhat as the technology develops further.
Another aspect of a desirable high K dielectric is in terms of its equivalent capacitance to that of a certain thickness of silicon oxide. Silicon oxide has been so commonly and effectively used that it has become a standard and the industry often describes certain characteristics in terms of its relationship to silicon oxide. In this case, the typical desirable silicon oxide equivalent thickness is between 5 and 15 angstroms but with silicon oxide of 5 to 15 angstroms it has problems with leakage, reliability, growth rate, and uniformity. Thus, when a film is that small there can be difficulties in manufacturing it as well as using it. The desirable coupling is to have a dielectric that has the equivalence of the thickness of 5 to 15 angstroms of silicon oxide but a greater actual thickness.
High K dielectric films which include aluminum have been developed, yet aluminum is known to cause high interface state density and degraded mobility in silicon based devices.
Thus, there is a need for a dielectric film which has a dielectric constant within a desirable range, the ability to be made of high integrity, a thickness in a desirable range, does not degrade mobility or cause high interface state densities, and has the ability to be made in a manufacturing process.